Most amplifiers consist of three stages:
1. An input stage, normally a differential voltage to current converter stage, in which current variations are relatively small; PA1 2. followed by an intermediate stage whose output may produce large voltage variations and about which the amplifier's dominant pole is often formed; PA1 3. which is then followed by an output stage most often in the form of a voltage follower.
All three stages produce distortion which affects the signal to be amplified, the output stage usually being the worst.
Throughout this specification the following descriptions are used:
The input signal to the amplifier is termed an "amplifier input signal" and is supplied to an "amplifier input", which is used to denote the input of an "input stage" adapted to amplify the amplifier input signal and to provide a "first signal."
The first signal is fed to a next stage, termed an "intermediate stage" which is adapted to amplify the said first signal and further adapted to provide a "second signal." The second signal is fed to an input of an "output stage" adapted to amplify the said second signal and further adapted to provide an "amplifier output signal" to an "amplifier output."
A "power supply means" is adapted to provide power to the input stage, the intermediate stage and the output stage.
A review of the current state of the art in low distortion has been given by D. Self in a series of articles in "Electronic World+Wireless World" from August 1993 to January 1994. In these articles distortions in each of the three stages as set out above are described and current best methods for achieving low distortion are discussed.
The D. Self articles discussed above recommended that the first stage is a differential voltage to current converter, followed by an intermediate stage about which the dominant amplifier pole is formed. This intermediate stage is a cascade amplifier feeding a constant current source so that distortion from the well known "Early effect" is avoided. Buffer transistors are inserted between the output transistors and the intermediate stage. The lowest distortion is achieved by using a symmetrical complementary bi-polar transistors in the output stage, where the one half is a driver-npn-cum-power-pnp pair configured with a large measure of local negative feedback and the other half is similarly a driver-pnp-cum-power-npn pair configured with a large measure of local negative feedback. The Self articles also recommends and teach certain circuit layouts so that spurious sources of distortion causing mechanisms may be avoided.
The lowest combination of distortion as taught by those articles gives a total harmonic distortion of about 10 parts per million (10 ppm) at 1 kHz and a little over 100 ppm at 20 kHz. However this was measured in a amplifier of very modest power, namely 10 watts. These figures are thought to be consistent with the best state-of-the-art low distortion amplifiers presently commercially available and possible from currently known techniques.